You have committed the common beginner error of assuming that an Element's value is the text in it's only child node. It isn't. Read the API documentation for Node to find out what an Element's value actually is.

And then consider that an Element can have more than one child (as does the <abc> element in your example). If you want to change any of its child nodes (and you do want that) then you will either have to change them, if possible, or create new ones to replace the existing ones.